1. Field of the Invention
The present invention relates to an alternator driven by an internal combustion engine, and more particularly, to a rotor for the alternator mounted to a vehicle such as a passenger car and a truck.
2. Description of the Related Art
FIG. 12 is a cross-sectional view showing a conventional alternator for a vehicle. FIG. 13 is a perspective view showing a rotor for the conventional alternator for a vehicle.
In FIGS. 12 and 13, the alternator for a vehicle is constructed such that a Lundell-type rotor 7 is rotatably mounted within a casing 3 including an aluminum front bracket 1 and an aluminum rear bracket 2 by means of a shaft 6, and a stator 8 is fixedly attached to an inner wall surface of the casing 3 so as to cover an outer circumferential side of the rotor 7. Fans 5 are fixedly attached to both ends of the rotor 7 in its axial direction.
The shaft 6 is rotatably supported by the front bracket 1 and the rear bracket 2. A pulley 4 is fixedly attached to one end of the shaft 6 to enable rotational torque from an engine to be transmitted to the shaft 6 by means of a belt (not shown). Slip rings 9 for supplying electric current to the rotor 7 are fixedly attached to the other end portion of the shaft 6. A pair of brushes 10 are housed in a brush holder 11 disposed within the casing 3 so as to slide in contact with the slip rings 9. A regulator 18 for regulating the magnitude of an alternating voltage caused by the stator 8 is adhered to a heat sink 17 fitted to the brush holder 11. Further, a rectifier 12 electrically connected to the stator 8 and rectifying an alternating current caused by the stator 8 to a direct current is mounted into the casing 3.
The rotor 7 has a field winding 13 which generates magnetic flux when an electric current flows therein, and a pair of rotor iron cores 20 and 21 disposed so as to cover the field winding 13 in which magnetic poles are formed by the magnetic flux generated by the field winding 13. The rotor iron cores 20 and 21 are made of iron and each has disk-shaped basic portions 22a and 23a and plural claw-shaped magnetic poles 22b and 23b projecting from outer circumferential edges of the basic portions 22a and 23a at an equiangular pitch circumferentially. Central holes are bored in the basic portions 22a and 23a. Each of the claw-shaped magnetic poles 22b and 23b is approximately formed in a trapezoidal shape projecting in a tapered shape in an axial direction. The pair of rotor iron cores 20 and 21 are fitted to the shaft 6 and are integrated with the shaft 6 by abutting end surfaces of the basic portions 22a and 23a so as to engage the claw-shaped magnetic poles 22b and 23b with each other. Further, permanent magnets 19 are fixedly attached between the claw-shaped magnetic poles 22b and 23b adjacent to each other , respectively, and are magnetized in a direction in which leakage of the magnetic flux between these claw-shaped magnetic poles 22b and 23b is reduced.
The stator 8 has a stator iron core 15 and a stator winding 16 constructed by winding a lead wire around the stator iron core 15 in which the alternating current is generated by alternating the magnetic flux from the field winding 13 owing to the rotation of the rotor 7.
In the alternator for a vehicle having such a construction, an electric current is supplied from a battery (not shown) to the rotor coil 13 by means of the brushes 10 and the slip rings 9 so that the magnetic flux is generated. The claw-shaped magnetic poles 22b of one field core 20 are magnetized to N-polarities by the magnetic flux and the claw-shaped magnetic poles 23b of the other field core 21 are magnetized to S-polarities.
In contrast to this, the rotational torque from the engine is transmitted to the shaft 6 by means of the belt and the pulley 4 so that the rotor 7 is rotated. Therefore, a rotating magnetic field is imparted to the stator winding 16 and the electromotive force is generated in the stator winding 16. This alternating electromotive force is rectified to a direct current by means of the rectifier 12, and the regulator 18 regulates its magnitude and the direct current is charged to the battery.
Most of magnetic flux generated by the field winding 13 enter the stator 8 from the claw-shaped magnetic poles 22b of the field core 20 magnetized to the N-polarities, and then enter the interior of the field core 21 from the claw-shaped magnetic poles 23b of the field core 21 magnetized to the S-polarities through the interior of the stator 8, and again enter the stator 8 from the claw-shaped magnetic poles 22b of the field core 20. Thus, the above magnetic flux constitutes a closing circuit. At this time, magnetic flux leaked from a portion between the claw-shaped magnetic poles 22b and 23b are reduced by the permanent magnets 19. Thus, invalid magnetic flux not contributing to power generation is reduced and power generation efficiency of the alternator is increased.
Since the rotor 8 of this conventional alternator for a vehicle is constructed as mentioned above, the magnetic attractive force is applied to the claw-shaped magnetic poles 22b and 23b by the magnetic flux in gaps between the claw-shaped magnetic poles 22b and 23b and the stator 8 during an operation of the alternator for a vehicle. This magnetic flux is continuously changed with the passage of time, and the magnetic attractive force is applied to the claw-shaped magnetic poles 22b and 23b as swinging force. Thus, as shown by an arrow in FIG. 14, a problem exists in that the claw-shaped magnetic poles 22b and 23b are swung, i.e., resonated so that uncomfortable noises (electromagnetic noises) are caused and heard.
Further, the claw-shaped magnetic poles 22b and 23b are vibrated by the magnetic attractive force with large amplitude as it approaches tip sides of the claw-shaped magnetic poles 22b and 23b. The claw-shaped magnetic poles 22b and 23b are mutually vibrated in reverse phases. In the conventional rotor 8, the permanent magnets 19 are fixedly attached to the claw-shaped magnetic poles 22b and 23b and are constructed as an integral body. Accordingly, a problem also exists in that there is a fear that the permanent magnets 19 fixedly attached to the claw-shaped magnetic poles 22b and 23b are distorted and damaged by such vibration of the claw-shaped magnetic poles 22b and 23b.
For example, improving measures for preventing such damage of the permanent magnets are proposed in Japanese Patent Application Laid-Open No. Hei 11-136913. In these improving measures, each permanent magnet interposed between the claw-shaped magnetic poles adjacent to each other is divided into two permanent magnet pieces, and the divided permanent magnet pieces are respectively directly fixedly attached to the adjacent claw-shaped magnetic poles, or are respectively fixedly attached to the adjacent claw-shaped magnetic poles by using supporting members.
However, in such improving measures, strength of the magnet is secured, but these improving measures are not positively taken to reduce the resonance of each of the claw-shaped magnetic poles. Accordingly, no improving measures contribute to a reduction in displacement of each of the claw-shaped magnetic poles so that no uncomfortable noises can be reduced. Further, when the permanent magnet pieces are fixedly attached to the claw-shaped magnetic poles by using the supporting members, the supporting members, the permanent magnet pieces and the claw-shaped magnetic poles are integrated with each other so that only volume is simply increased. Therefore, the vibration of the claw-shaped magnetic poles is ununiformly dispersed by rotation so that there is also a fear that the rasping noises become bigger. Further, the alternator is set to a state in which weight is loaded while no displacement of the claw-shaped magnetic poles is reduced. Therefore, there is also a fear that strength improvement of the permanent magnet pieces using the supporting members becomes imperfect at a high speed rotating time.